1. Field of the Invention
This invention relates generally to production of metal particles through electrolysis, and more specifically, to a system for controlling the concentration of the electroreducible metal species to optimize the quality of metal particles produced through electrolysis.
2. Related Art
There are many applications for metal particles produced through electrolysis including, for example, for use as feedstock for laboratory and industrial processes, and for use in refuelable and regenerative metal/air fuel cells. In these fuel cells, the metal particles function as the fuel for replenishing discharged fuel cells, and this fuel can be regenerated from the spent reaction solution which results from fuel cell discharge. In applications such as this, it is desirable to be able to regenerate the metal particles in a space efficient and self contained manner so that the regeneration of the metal particles can take place at the same location as the power source or cell stack within the fuel cell. For additional information on metal/air fuel cells, the reader is referred to the following patents and patent applications, which disclose a particular embodiment of a metal/air fuel cell in which the metal is zinc: U.S. Pat. Nos. 5,952,117; 6,153,328; and 6,162,555; and U.S. patent application Ser. Nos. 09/521,392; 09/573,438; and 09/627,742, each of which is incorporated herein by reference as though set forth in full. The term “fuel cell” as used throughout this disclosure is synonymous with the terms “battery” and “refuelable metal/air battery.”
Unfortunately, known methods of producing metal through electrolysis are all unsatisfactory for these applications. Some methods, e.g., electroplating, do not produce metal in the required particulate form, and require expensive and cumbersome mechanical processing to put the metal in the required form.
For example, a method disclosed in U.S. Pat. No. 4,164,453 forms zinc dendrites on cathode tips that protrude into an anodic pipe carrying a flow of zincate solution. The cathode protrusions are specially formed in a curved configuration. Dendrites form on the cathode tips during low flow in one direction, and are then dislodged during high flow in the opposite direction. This technique is not suitable for particle production because it yields dendritic zinc that requires further processing to make pellets. Also, the curved cathodic protrusions are expensive to manufacture, and spatially inefficient.
Another method, represented by U.S. Pat. No. 5,792,328, involves electro-depositing dendritic or mossy zinc onto the surface of a planar cathode plate, and then scraping the zinc from the surface of the cathode. Since the recovered metal is in the form of mossy dendrites, and cannot be easily put into the desired particulate form absent expensive and complicated mechanical processing steps, this method is likewise not suitable.
A third method, in U.S. Pat. No. 3,860,509, uses a cathodic surface that consists of many small conductive areas in the hundred micron range spaced apart by an insulating matrix. These areas are exposed to a high temperature metal bearing electrolyte solution which, by electrolysis, deposits metal dendrites on the cathode. The metal is recovered by mechanically scraping the cathode which produces a powdery metal dust composed of particles so small that they are not suitable for use in a metal/air fuel cell.
A fourth method, known as electrowinning, represented by U.S. Pat. Nos. 5,695,629 and 5,958,210, involves immersing seed particles in an electrolyte, and causing metal to form over the seed particles through electrolysis. However, because of the risk that metal particles will get caught in a porous separator between the anode and cathode, and cause a disastrous short between the anode and cathode, this method is unsatisfactory. Another factor weighing against this method is the burden and expense of maintaining a supply of seed particles.
Another method, represented by U.S. Pat. No. 5,578,183, involves forming dendritic or mossy metal on a cathode through electrolysis, removing the metal, and then pressing the metal into pellets through mechanical forming steps such as extrusion. This technique is unsuitable for the applications mentioned earlier because the required mechanical forming steps are expensive, and do not permit a space-efficient and self-contained particle recovery process.